1. Field of the Invention
The present invention relates generally to a cutter knife suited for cutting a relatively soft work material, e.g. cardboard. In particular, the present invention relates to a safety cutter knife, in which the blade returns automatically into the housing at the moment when the cutting operation is completed.
2. Description of the Related Art
Japanese Patent No. 3,409,177 (corresponding to U.S. Pat. No. 6,148,520) discloses this type of a cutter knife.
The safety cutter knife disclosed in Japanese Patent No. 3,409,177 is highly safe in use. In normal use, the cutter knife is slightly inclined with respect to the surface of a sheet, the blade extended from the housing is thrust into the sheet, and the cutter knife is pulled rearward to cut the sheet. At this time, the blade is further extended by the cutting friction resistance of the sheet, whereby a locking mechanism inside the housing is released. After the cutting operation is completed, that is, when the blade is released from the cutting friction resistance, the blade returns automatically by virtue of the energizing force of a spring.
However, in this kind of automatic return mechanism, depending on use conditions, the blade sometimes does not return automatically at the time when the cutting operation is completed. For example, in the case the cutting edge of the blade extended from the housing is placed on a fringe of a sheet of cardboard at right angles, and the cutter knife is drawn downward along the surface of the cardboard to cut the same with the attitude of the cutter knife being kept, the extended blade sometimes does not return automatically when the cutting operation is completed. This is because, in such the manner of cutting operation, a force for further extend the blade is not generated, and thus the automatic return mechanism does not operate.
The various manner of cutting operations will be conducted by the user, as the situations demand. A safer cutter knife applicable various cutting manner is expected to be developed. The technical problem in the prior art automatic return mechanism lies in that the automatic return mechanism is operative, only when the blade is further extended by the friction resistance of the work material.